This invention relates to streaming cartridge tape drives, and more particularly to a streaming cartridge tape drive incorporating a data smoother for removing phase and certain frequency variations in the data signal read from magnetic tape by the tape drive.
Streaming cartridge tape drives provide mass storage of magnetic information by writing long streams of serial data in a plurality of parallel streams on magnetic tape. The magnetic tape includes many such parallel data streams, each stream being written at a different vertical position on the tape. When the tape drive completes writing a serial data stream along the entire length of the tape at one vertical position on the tape, the tape drive reverses the direction of the tape and writes another serial data stream at a second vertical position on the tape. This back and forth method of recording data in a plurality of parallel streams on magnetic tape is called serpentine recording, and is well known in the streaming cartridge tape drive art.
Serpentine recording is advantageous in that it maximizes the amount of magnetic tape utilized by eliminating tape starts and stops inherent in tape drives along the entire width of the tape. To achieve high data transfer rates, streaming cartridge tape drives have relatively high speed transport systems which transport the magnetic tape past the magnetic heads at rates of speed of up to 90 inches per second.
Magnetic information is recorded on the tape as a series of magnetic poles, the transition between each differing magnetic pole representing a bit of information. The magnetic poles are produced by providing an alternating write current to the magnetic write head. The write current is first provided in one direction in the winding of the magnetic head to produce a magnetic pole and then in the opposite direction in the winding to produce an opposite magnetic pole on the tape. Although the magnetic write heads are energized by the tape drive at a uniform rate, the magnetic transitions are not written to the tape at uniform positions for a number of reasons.
First, the speed at which the magnetic tape passes by the write head varies. This speed variation is caused in part by speed variation in the transport system which moves the tape as well as by stretching of the tape in response to the varying tension applied by the tape transport system. As a result of this tape speed variation, the frequency of the magnetic transitions written to the tape varies. This variation is of relatively low frequency, generally ranging from as low as 50 Hz to approximately 5000 Hz at the high end.
Another factor which contributes to the nonuniform spacing of magnetic transitions on the tape is a phenomenon known as "peak shift." Peak shift, which results from magnetic interference effects when the magnetic transitions are recorded, is a relatively high frequency effect, with a frequency generally on the order of the nominal data rate of the tape drive. In addition, noise also contributes to high frequency variation of the data on the tape.
Data recovery systems for retrieving the data from the tape after it has been recorded generally include some type of data smoother in order to reduce the high frequency variations in the data signal induced by peak shift. However, the data smoother preserves the relatively low frequency data variations induced by the tape transport system so that no complex data input/output buffering system is required in the data smoother. Thus, the frequency at which data leaves the data smoother generally tracks the frequency at which data enters the data smoother, except that high frequency variations due to peak shift and noise are removed from the data signal prior to being output by the data smoother.
To accomplish the foregoing, this type of data smoother generates an internal clock signal which tracks the low frequency variations in the data signal caused by the tape transport system, but ignores the relatively high frequency peak shift variations. This clock signal, which is used to control the rate at which data is output from the data smoother, is usually generated by an oscillator. The oscillator is generally coupled to some type of phase comparator which is used to vary the oscillator's rate of oscillation based upon the phase difference detected between the data signal input to the data smoother and the internal clock signal.
Various methods have been used to vary the frequency of the clock signal based upon the detected phase difference between the data and clock signals. For example, in one method, each leading edge of the data signal input to the data smoother is delayed by a fixed amount and then compared with the trailing edge of a clock signal generated by an oscillator in order to determine whether to increase or decrease the frequency of its oscillation. This fixed delay of the leading edge of the data signal, which is based upon the nominal data rate of the system, causes this type of data smoother to adjust the frequency of the clock signal even though it may correspond to the actual data rate.
In other methods of adjusting the frequency of the clock signal, a digital counter may be used to detect the amount of phase difference by generating a count which relates to the phase difference. However, since only a discrete number of counts are provided by the counter, the sensitivity with which the phase difference is detected is limited, and thus the output of the counter is not directly proportional to the actual phase difference. As a result, the frequency of the clock signal is not adjusted by an amount directly proportional to the actual phase difference.